Update TCP_common.c

[xzhdream]

func: add_priority
priority_queue_start is a list for read, priority_queue_end is just as a tag

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 10.00%. Comparing base (a1e2458) to head (79d1bbf).
Report is 269 commits behind head on master.

Files with missing lines	Patch %	Lines
toxcore/TCP_common.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2387      +/-   ##
==========================================
+ Coverage    9.98%   10.00%   +0.02%     
==========================================
  Files         141      141              
  Lines       32530    32530              
==========================================
+ Hits         3247     3255       +8     
+ Misses      29283    29275       -8     

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[iphydf]

Can you explain a bit more what this change is doing?

[xzhdream]

Can you explain a bit more what this change is doing?

it is only a logical problem. priority_queue_start is used to read.
if TCP_Priority_List *p = con->priority_queue_start
then if (p != nullptr) { p->next = new_list; } else { con->priority_queue_start = new_list; }
can work as its goal

[JFreegman]

@xzhdream How does this change current behaviour? As far as any of us are aware, this function already works as intended. Are we mistaken?